Intercommunication system



Sept. 16, 1947. w. FELDSCHER INTERCOMMUNICATION SYSTEM Filed Oct. 29, 19.5 4 Sheets-Sheet 1 lb III Av wmfi Z: 1 II II Z; Q- wwmo. N: F0- I I I lIII III LIL I NH E I 2 I INVENTOR.

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WILLARD FELDSCHER Sept. 16, 1947. w. FELDSCHER INTERCOMMUNICATION SYSTEMFiled Oct. 29, 1945 4 Sheets-Sheet 4 INVENTOR. WILLARD F'ELDSCHERPatented Sept. 16, 1947 ZAZ'ZAit INTERCOMIVIUNICATION SYSTEM WillardFeldscher, San Mateo, Calif., assignor to The Goldfield ConsolidatedMines Company, San Francisco, Calif., a corporation Application October29, 1945, Serial No. 625,152

12 Claims.

Thi invention relates to intercommunicating apparatus in general, andmore particularly to intercommunicating systems comprising a pluralityof stations; each station including a transducer usable both as atransmitting microphone and as a loud-speaking receiver.

An object of this invention is to provide an improved intercommunicatingsystem comprising a plurality of stations; each stationincluding atransducer operable either as a transmitting microphone or as aloud-speaking receiver and an amplifier.

Another object of this invention is to provide an improvedintercommunicating system comprising a plurality of stations; eachstation including a transducer operative selectively as a transmittingmicrophone and as a loud-speaking receiver, and an amplifier adapted tofunction as a voltage amplifier when the associated transducer functionas a microphone and to function as a power amplifier when the associatedtransducer functions as a loud-speaking receiver.

A further object of this invention is to provide an intercommunicatingsystem comprising a plurality of stations; each station including atransducer operative selectively as a transmitting microphone and as aloud-speaking receiver and an amplifier; the amplifier at each stationcomprising only a part of the total amplification available to thatstation.

Still another object of this invention is to provide anintercommunicating system comprising a plurality of stations, each ofthe stations having an amplifying device, and connections between theamplifying devices, whereby the amplifying devices are enabled tofunction as parts of an amplifying unit during communication between thestations.

Other and further objects will be apparent to those skilled in the artto which this invention relates from the following specification anddrawings.

Referring to th drawings briefly, Figs; 1 and 2, taken together,illustrate an intercommunicating system embodying the present invention;Fig, 1 illustrating diagrammatically the apparatus disposed at a stationA incorporated in the system, and Fig. 2 illustrating diagrammaticallythe apparatus disposed at a station B incorporated in the system.

Figs. 3 and 4, taken together, illustrat a modified form of theintercommunicating system embodying the present invention; Fig. 3illustrating diagrammatically the apparatus disposed at a station #1incorporated in the system, and Fig. 4

illustrating diagrammatically the apparatus disposed at a station #2incorporated in the system.

Referring now more particularly to Figs. 1 and 2 of the drawings, theintercommunicating system there illustrated comprises a plurality ofstatlons including the stations A and B, provided with apparatusrespectively illustrated in Figs. 1 and 2. More particularly, theapparatus disposed at the station A comprises a transducer IOI operativeselectively as a transmitting microphone and as a loud-speakingreceiver; a combination rectifier and amplifier tube I02; a talklistenkey I03; a master switch I00 for connecting th tube I02 to a source of-volt A. C. power supply; and an electric network including a couplingtransformer I05 coupling the apparatus at the station A to an associatedsignalling line I00 extending between each of the stations A, B, etc.

Preferably, the transducer IOI is of any suitable type such, forexample, as the electrodynamic or the piezoelectric type; while the tubeI02 is of the 117L7GT type incorporating 9, rectifier portion (includingan anode I00, a cathode I0! and a cathode heater I03) and a beam powertetrode portion (including an anode I09, a screen grid H0, a controlgrid III, a cathode II2, beam electrodes I I3 and a cathode heat H4) Inthe tube I02, the tetrode portion constitutes a voltage amplifier whenthe transducer IOI is functioning as a microphone, and constitutes abeam power amplifier when the transducer IOI functions as aloud-speaking receiver. Also in the tube I02 the rectifier and amplifierportions ar enclosed in a common envelope I I 5; while the cathodeheaters I 08 and H4 are connected in series to the master switch I04.

The talk-listen key I03 is of the conventional four-blade double-throwtype, selectively operative between talk and listen positions.Specifically, the talk-listen key I03 comprises four blades IIB, II I,H8 and H9 that are operative together and normally biased to the listenposition.

Considering the connection and arrangement of the circuit network ingreater detail, one terminal of the transducer IOI is connected througha shielded cable I20 to th blade II6 of the talklisten key I03, and theother terminal of this transducer is connected to the blade II8 of thetalk-listen key I03 and to the by-pass capacitor I2 I. The make contactassociated with the blade H5 and the break contact associated with theblade I I! are connected to the control grid I II of the tetrodeamplifier through the shielded cable I22, which is also connected to thegrid leak resistor I23. The sheaths of both of the shielded cables I20and I22 are grounded to the chassis of the apparatus to reduce straypickup in the grid circuit of the tetrode amplifier. The break contactof the blade H6 and the make contact of the blade II! are connected tothe screen grid IIO of the tetrode and to the filter capacitor I24 andresistor I25 through the conductor I3I. The make contact of the blade H8and the break contact of the blade I I9 are connected to the bus I26;which is connected to one blade of the master power switch I04, to bothof the filter capacitors I24 and I21, and to a terminal of the bypasscondenser I28, the other terminal of this latter condenser beinggrounded to the chassis. The cathode resistor I29 and the cathoderesistor shunting capacitor I30 are also connected to the bus I20,together with the lower terminal of the grid leak resistor I23.

The break contact of the blade H8 and the make contact of the blade II9are connected to the anode I09 of the tetrode amplifier; and the bladesIII and H9 are connected to the primary winding of the couplingtransformer I05, the secondary winding of which is connected to the'gnalling line I00. The coupling transformer I comprises a portion of animpedance network for matching the apparatus disposed at the station Ato the signalling line I00. More particularly, both the transducer I01and the coupling transformer I05 are adapted to be matched both to theinput and to the output of the tetrode amplifier, as explained morefully hereinafter. Specifically, in the circuit control network disposedat the station A it has been found that the following values for thedifferent elements are quite satisfactory:

Filter capacitor I24 mfd Filter capacitor I21 mfd Filter resistor I25ohms By-pass capacitor I28 mfd Cathode resistor I29 ohms Cathoderesistor shunting capacitor I30 mid" 25 Grid leak resistor I23 ohms250,000 By-pass capacitor I2I mfd 0.005

Similarly, the apparatus disposed at the sta: tion B comprises atransducer 20I operative selectively as a transmitting microphone and asa loud-speaking receiver; a combination rectifier and amplifier tube202; a talk-listen key 203; a master switch 204 for connecting the tube202 to a source of 110-volt A. C. power supply; and an electric networkincluding a. coupling transformer 205 coupling the apparatus at thestation B to an associated signalling line I00 extending between each ofthe stations A, B, etc.

Preferably, the transducer 20I is of any suitable type such, forexample, as the electrodynamic or the piezo-electrio type; while thetube 202 is of the 11"IL'7GT type incorporating a rectifier portion(including an anode 200, a cathode 201 and a cathode heater 208) and abeam power tetrode portion (including an anode 209, a screen rid 2I0, acontrol grid 2| I, a cathode 2I2, beam electrodes 2 I3 and a, cathodeheater 2 I4) In the tube 202, the tetrode portion constitutes a voltageamplifier when the transducer 20I is functioning as a microphone, andconstitutes a beam power amplifier when the transducer 20I functions asa loud-speaking receiver. Also in the tube 202 the rectifier andamplifier portions are enclosed in a common envelope 2I5; while thecathode heaters 208 and 2I4 are connected in series to the master switch204.

The talk-listen key 203 is of the conventional four-blade double-throwtype, selectively operative between talk and listen positions.Specifically, the talk-listen key 203 comprises four blades 2I6, 2I'I,2I8 and 2I9 that are operative together and normally biased to thelisten position.

Considering the connection and arrangement of the circuit network ingreater detail, one terminal of the transducer 20I i connected through ashielded cable 220 to the blade 2I6 of the talklisten key 203, and theother terminal of this transducer is connected to the blade MB of thetalk-listen key 203 and to the by-pass capacitor 22 I. The make contactassociated with the blade 2H5 and. the break contact associated with theblade 2H are connected to the control grid 2I I of the tetrode amplifierthrough the shielded cable 222, which is also connected to the grid leakresistor 223. The sheaths of both of the shielded cables 220 and 222 aregrounded to the chassis of the apparatus to reduce stray pickup in thegrid circuit of the tetrode amplifier. The break contact of the blade 2I6 and the make contact of the blade 2 H are connected to the screengrid 2I0 of the tetrode and to the filter capacitor 224 and resistor 225through the conductor 23L The make contact of the blade 2 I8 and thebreak contact of the blade 2 I 9 are connected to the bus 226; which isconnected to one blade of the master power switch 204, to both of thefilter capacitors 224 and 221, and to a terminal of the by-passcondenser 228, the other terminal of this latter condenser beinggrounded to the chassis. The cathode resistor 229 and the cathoderesistor shunting capacitor 230 are also connected to the bus 226,together with the lower terminal of the grid leak resistor 223.

The break contact of the blade 2I8 and the make contact of the blade 2I9are connected to the anode 209 of the tetrode amplifier; and the blades2H and 2H! are connected to the primary winding of the couplingtransformer 205, the secondary winding of which is connected to thesignalling line I00. The coupling transformer 205 comprises a portion ofan impedance network for matching the apparatus disposed at the stationB to the signalling line I00. More particularly, both the transducer 20Iand the couplin transformer 205 are adapted to be matched both to theinput and to the output of the tetrode amplifier, as explained morefully hereinafter. Specifically, in the circuit control network disposedat the station B it has been found that the following values for thedifierent elements are quite satisfactory:

Filteg gapacitor 224 mfd 40 Filter capacitor 221 mfd 20 Filter resistor225 ohms 500 By-pass capacitor "228 mfd 0.1 Cathode resistor 229 oh1ns500 Cathode resistor shunting capacitor 230 mfd 25 Grid leak resistor223 ohms 250,000 By-pass capacitor 22I mfd 0.005

Considering now the operation of the intercommunicating system, when theperson at the station A wishes to communicate with the person at anyother station connected to the system, for example the person at thestation B, he merely operates the talk-listen key I03 from its normallistening position to its operated talk position, as illustrated inFig. 1. This operation of the talk-listen key I03 at the station Acauses the transducer IOI'to be operative as a transmitting microphoneinstead of as a loud-speaking receiver, and causes operation of thetetrode amplifier as a first-stage voltage amplifier instead of as asecond-stage power amplifier. It is further assumed that at this timethe master switch I04 at the station A occupies its closed position andthat the'master switch at the other station, such for example a themaster switch 284 at the station B, occupies its closed position.

When the master switch HM at the station A is closed, the source of110-volt alternating current power supply is connected directly acrossthe series-connected cathode heaters I08 and I I4, whereby the heatersmentioned effect heating of the associated cathodes I01 and H2 in orderto render these cathodes electron emissive. Also, closing the masterswitch I04 applies the 110- volt alternating current to the anode I06 ofthe rectifier. When the positive half-cycle of the alternating currentis applied to this anode I06 current flows between the cathode Hi7 andthe anode Hi6, thereby charging the capacitor 121; and at the same timesome of this current flows through the resistor 82-5, also partiallycharging the capacitor 2 1. The current also at this time fiows throughthe conductor I3! applying a positive potential to the screen grid I illof the tetrode; and thence through the blade iii of the talkliste-n keyI83 to the primary of the transformer Hi5; and from the primary of thistransformer E85 through the blade H9 or the talk-listen key 33 to theanode its of the tetrode amplifier, thereby rendering the amplifieroperative. When the negative half -cycle of the alternating current isapplied to the anode Hi6 of the rectifier, no current flows between thecathode ml and the anode Hi8; and during this half-cycle current for thetetrode amplifier is supplied by the capacitors in and i2? of the filternetwork, inasmuch as during this half-cycle these capacitors at leastpartially discharge, thereby applying a positive potential to theconductor I3! and to the screen grid Hill of the tetrode amplifier.Current from the conductor H31 is supplied to the primary of thetransformer Hi5 and to the anode I09 of the tetrode amplifier throughthe aforesaid blades i ii and l 59 of the talk-listen key i 63.

Inasmuch as the transducer it! connected to the grid H! of the tetrodeamplifier functions at this time as a microphone, modulated potentialsproduced by this transducer modulate the electron stream through thetetrode amplifier, and the aforesaid currents flowing from the conductorl3! through the blade ill of the talk-listen key M3, the primary of thetransformer H35, and the blade E E9 of the talk-listen key N33 to theanode of this tetrode amplifier are modulated by the action of the gridill of the tetrode amplifier in accordance with the sound vibrationspicked up by the transducer lei. These undulating electric currentsproduced by the operation of the voltage amplifier are applied to thesignalling line I90 by the transformer I05, and they are received fromthis line by the transformer 205 at the station B and the correspondingtransformers at the other stations.

Considering now the operation of the apparatus at one of the stationsoperating as a receiving station, such for example as the station B, theamplified undulating currents transmitted over the signalling line Hilleffect operation of the coupling transformer 205, the primary winding ofwhich is connected to the blades 2!! and M9 of the talk-listen key 203.More particularly, at this time it is assumed that the talk-listen key283 occupies its normal listen position and that the power switch 204occupies its closed position, whereby the undulating currents are fedover the shielded conductor 222 connected to the break contact of theblade 2|! to the control grid 2!! of the tetrode amplifier. Undulatingcurrents are also fed from the break contact of the blade 6 2| 9 to thecathode of the tetrode amplifier by way of the conductor 226 and thecathode resistor .229 and cathode resistor shunting condenser 230. Theseundulating currents are amplified by the tetrode amplifier through theoperation of the control grid 2 and its influence on the electron streambetween the cathode 2l2 and the anode 209, so that correspondingundulating currents of an amplified order are fed from the anode 209 ofthe tetrode amplifier through the break contact of the blade 218 to oneterminal of the transducer Zfil which is connected to the blade H8. Theother terminal of the transducer 20] is connected to the blade 216 andthrough the break contact of this blade -2l6 to the conductor 23l, whichis connected to the screen grid electrode 210 of the tetrode amplifierand to the filter network consisting of the capacitors 224 and 221 andthe resistor 225; whereby the transducer 20!, functioning as aloud-speaking receiver, reproduces sounds corresponding to theundulating currents fed thereto.

The rectifier, consisting of the anode 206, the cathode 20! and thecathode heater 2H8, functions in the same manner as the correspondingrectifier disposed at the station A and illustrated in Fig. 1, in thatduring the interval when the positive half-cycle is applied to the anode206, current flows between the cathode 201 and the anode 2136, therebycharging the capacitor 221; and at the same time, some of this currentflows through the resistor 225, also partially charging the capacitor224. The current also at this time flows through the conductor 23! asdescribed above. Thus, the transducer 201 functions as a loud-speakingreceiver and the tetrode amplifier functions as a beam power amplifier.

Of course it will be understood that in the intercommunicating systemafter the person at the station A is through talking he may restore theassociated talk-listen key 163 to its normal listen position, wherebythe station A is converted to a receiving station; and at this time theperson at the station B may operate the associated talklisten key 283 toits talk position, whereby the station B is converted to a transmittingstation. Further, it will be understood that when any one of thestations is operating as a transmitting station, the associatedtransducer functions as a transmitting microphone and the associatedtetrode amplifier functions as a voltage amplifier; whereas when any oneof the stations is operating as a receiving station, the associatedtransducer functions as a loud-speaking receiver and-the associatedtetrode amplifier functions as a beam power amplifier. Finally, it willbe appreciated that only one station at any one time may operate as atransmitting station, whereas all of the other stations may operatesimultaneously as receiving stations.

Referring now more particularly to Figs. 3 and 4 of the drawings, themodified form of the intercommunicating system there illustratedcomprises a plurality of stations, including the station #1 and thestation #2, provided with apparatus respectively illustrated in Figs. 3and 4. More particularly, the apparatus disposed at the station #1comprises a transducer 3! operative selectively as a transmittingmicrophone and as a loud-speaking receiver; a combination rectifier andamplifier tube 382; a talklisten key 303; a master switch 304 forconnecting the tube 382 to a source of volt A. C. power supply; animpedance matching network including a station selector switch 395; anda 7 home signalling line 35!, accessible to the selector switches ateach of the other stations.

Preferably, the transducer 3M is of any suitable type, such for exampleas the electrodynamic or the piezoelectric type; while the tube 302 isof the 11'7P'7GT type incorporating a rectifier portion (including ananode 306, a cathode 381, and a cathode heater 308) and a beam powertetrode portion (including an anode 309, a screen grid 3l0, a controlgrid 3, a cathode 3|2, beam electrodes 3| 3 and a cathode heater 3M). Inthe tube 302, the tetrode portion constitutes a voltage amplifier whenthe transducer 30I is functioning as a microphone, and constitutes abeam power amplifier when the transducer 395 is functioning as a,loud-speaking receiver. Also, in the tube 302 the rectifier andamplifier portions are enclosed in a common en velcpe 3R5; while thecathode heaters 398 and 3H5 are connected in series to the master switch394.

The talk-listen key 383 is of the conventional two-blade double-throwtype, selectively operative between talk and listen positions.Specifically, the talk-listen key 393 comprises two blades 316 and 3Vthat are operative together and nor mally biased to the listen position.

Considering the connection and arrangement of the circuit network ingreater detail, one terminal if the transducer 3! is connected to theblade 35? of the talk-listen key 3% and the other terminal of thistransducer is connected through the bus 326 and the cathode resistor 329and cathode resistor shunting capacitor 336, to the cathode 3H2, of thetetrode amplifier.

The make contact associated with the blade SIS and the break contactassociated with the blade 3H are connected through the impedance network(consisting of the resistor 35!, coupling capacitor $62, and grid leakresistor 3&3) to the control grid 3!] of the tetrode amplifier. Thebreak contact of the blade 31S and the make contact of the blade 3H areconnected together and through a coupling capacitor 364 to the anode 389of the tetrode amplifier. 'The anode 389 of the tetrode amplifier isconnected to the screen grid 358 of the tetrode amplifier through theresistor which in turn is connected to the filter network consisting ofthe filter capacitors 324 and 32'! and the filter resistor 325. The terresistor 325 and the filter capacitor 32'? are connected to the cathode397 of the rectifier. The blade of the talk-listen key 383 is connectedthrough the conductor 33! to the capacitor SH of the impedance matchingnetwork consisting of capacitors 3H and 3'52 and resistor 31%. Thecapacitor 372 is connected to the bus 326, which is also connected toone side of the master power switch 366. The other side of the masterpower switch is connected to one terminal of the series-connectedcathode heaters 388 and 3 i i and to the rectifier anode 328, the otherterminal of the series-connected cathode heaters 388 and 354 beingconnected to the bus 326.

Thus the busses 33! and 32 are respectively connected by way of thecoupling capacitors 31! and 312 to the terminals of the resistor 3'13,which in turn are connected to the line conductors of the home line 35!and to the respective wipers of the station selector switch the stationselector switch 395 is of the conventional rotary type including tworotary wipers 88! and 382provided with individually associated contactbanks. The contacts in the contact banks respectively associated withthe rotary wipers 38l and 382 of the station selector switch 305 arepaired and terminate the corresponding line conductors of the home linesextending to the other stations #2, etc. The impedance network includingthe coupling capacitors 3H and 3'12 and the resistor 373 matches theapparatus disposed at the station #1 to the home line 35! and to any ofthe home lines extending to the other stations #2, etc, selected by wayof the associated station selector switch 365. More particularly, boththe transducer 3il and the impedance network mentioned are adapted to bematched both to the input and to the output of the tetrode amplifier, asexplained more fully hereinafter.

Similarly, the apparatus disposed at the station #2 comprises atransducer GU! operative selectively as a transmitting microphone and asa loud-speaking receiver; a combination rectifier and amplifier tube3E2; a talk-listen key 593; a master switch ie for connecting the tube632 to a source of volt A. C. power supply; an impedance matchingnetwork including a station selector switch 465; and a home signallingline t5l, accessible to the selector switches at each of the otherstations.

Preferably, he transducer 35! is of any suitable type, such for exampleas the electrodynamic or the piezo-electric type; while the tube Q82 isof the 11'7P7GT type incorporating a rectifier portion (including ananode 496, a cathode 407, and a cathode heater 488) and a beam powertetrode portion (including an anode 499, a screen grid Me, a controlgrid 5H, a cathode H2, beam electrodes M3 and a cathode heater 456), Inthe tube 662, the tetrode portion constitutes a voltage amplifier whenthe transducer 40! is functioning as a microphone, and constitutes abeam power amplifier when the transducer 433i is functioning as aloud-speaking receiver, Also, in the tube 182 the rectifier andamplifier portions are enclosed in a common envelope 415; while thecathode heaters 663 and 414 are connected in series to the master switch494.

The talk-listen key 393 is of the conventional two-blade double-throwtype, selectively operative between talk and listen positions.Specifically, the talk-listen key 593 comprises two blades M6 and lii'ithat are operative together and normall biased to the listen position.

Considering the connection and arrangement of the circuit network ingreater detail, one terminal of the transducer Gill is connected to theblade Gil of the talk-listen key 493 and the other terminal of thistransducer is connected through the bus 426 and the cathode resistor 429and cathode resistor shunting capacitor 136, to the cathode 3 i 2 of thetetrode amplifier.

The make contact associated with the blade 6 l 6 and the break contactassociated with the blade dil are connected through the impedancenetwork (consisting of the resistor 36i, coupling capacitor M2 and gridleak resistor 453) to the control grid HI of the tetrode amplifier. Thebreak contact of the blade MS and the make contact of the blade M7 areconnected together and through a coupling capacitor ,64 to the anode 469of the tetrode amplifier. The anode 489 of the tetrode amplifier isconnected to the screen grid M0 of the tetrode amplifier through theresistor 655, which in turn is connected to the filter networkconsisting of the filter capacitors E24 and t2? and the filter resistor(#25. The filter resistor 325 and the filter capacitor 821 are connectedto the cathode 40'! of the rectifier. The blade MB of the talk-listenkey 403 is connected through the conductor 33i to the capacitor 4?! ofthe impedance matching network consisting of capacitors All and 412 andresistor 473. The capacitor 412 is connected to the bus 426, which isalso connected to one side of the master power switch 404. The otherside of the master power switch AM is connected to one terminal of theseries-connected cathode heaters 483 and il l and to the rectifier anode406, the other terminal of the series-connected cathode heaters 468 and4M being connected to the bus 426 Thus the busses 43! and 426 arerespectively connected by way of the Coupling capacitors All and 472 tothe terminals of the resistor 413, which in turn are connected to theline conductors of the home line lEl and to the respective wipers of thestation selector switch 405; the station selector switch 485 is of theconventional rotary type includin two rotary wipers 48! and 482 providedwith individually associated contact banks. The contacts in the contactbanks respectively associated with the rotar wipers Gill and 682 of thestation selector switch 465 are paired and terminate the correspondingline conductors of the home lines extending to the other stations #1,etc. The impedance network including the coupling capacitors 4H and 412and the resistor M3 matches the apparatus disposed at the station #2 tothe home line 45! and to any of the home lines extending to the otherstations #1, etc., selected by way of the associated station selectorswitch 695. More particularly, both the transducer 49! and the impedancenetwork mentioned are adapted to be matched both to the input and to theoutput of the tetrode amplifier, as explained more fully hereinafter.

Considering now the operation of the intercommunicating system, when theperson at the station #1 wishes to communicate with the person at anyother station connected to the system, for example the person at thestation #2, he first operates the talk-listen key 3&3 from its normallistening position to its operated talk position, as illustrated in Fig.3. This operation of the talk-listen key 383 at the station #1 causesthe transducer 39! to be operative as a transmitting microphone insteadof as a loud-speaking receiver, and causes operation of the tetrodeamplifier as a first-stage voltage amplifier instead of as asecond-stage power amplifier. The person at the station #1 then operatesthe station selector switch 355 to the position corresponding to thedesired station, such for example as to the second position,corresponding to the station #2.

It is further assumed that at this time the master switch 394 at thestation #1 occupies its closed position and that the master switch ateach of the other stations, such for example as the master switch 464!at the station #2, occupies its closed position, When the master switch3% at the station #1 is closed, the source of 110-volt alternatingcurrent power supply is connected directly across the series-connectedcathode heaters 308 and 354, whereby the heaters mentioned efiectheating of the associated cathodes 38'! and 3I2 in order to render thesecathodes electron emissive. Also, closing the master switch 3115 appliesthe 1l0-volt alternating current to the anode 3535 of the rectifier.When the positive half-cycle of the alternating current is applied tothis anode 3%, current flows between the oathode 323? and the anode 3%,thereby charging the capacitor 321; and at the same time some of thiscurrent flows through the resistor 325, also partially charging thecapacitor 324. Also, the rectified current applies a positive potentialto the screen grid SIB of the tetrode amplifier, and current flowsthence through the resistor 355 to the anode 399 of the tetrodeamplifier, thereby rendering the amplifier operative. When the negativehalf-cycle of the alternating current is applied to the anode 356 of therectifier, no current flows between the cathode 30'! and the anode 3G6;and during this half-cycle current for the tetrode amplifier is suppliedby the capacitors 324 and 32'! of the filter network, inasmuch as duringthis half-cycle these capacitors at least partially discharge, therebyapplying a positive potential to the screen grid 3 l 0 and currentthrough the resistor 365 to the anode 309 of the tetrode amplifier.

Inasmuch as the transducer 39! connected to the blade 31'! of thetalk-listen key 303 is coupled by way of the capacitor 362 to thecontrol grid 3 of the tetrode amplifier, modulated potentials producedby this transducer modulate the electron stream through the tetrodeamplifier. The modulated currents in the output of the tetrode amplifierare impressed by way of the coupling capacitor 364, the blade 316 of thetalk-listen key 3%, and the associated break contact, upon the conductor33!. Thus the output of the tetrode amplifier is applied between the bus326 and the conductor 33L which are connected to the capacitors 31! and312 across the wipers 38] and 882 of the station selector switch 305 andconsequently across the line conductors of the selected home lineextending to the station #2. Accordingly, the transducer 3M is operativeas a transmitting microphone to control the tetrode amplifier, wherebyvoltage amplified undulating signal currents are transmitted over thestation selector switch 305 and the selected home line extending to theapparatus at the sta tion #2.

Considering now the operation of the apparatus at one of the stationsoperating as a receiving station, such for example as the station #2,the amplified undulating currents transmitted over the associated homeline 45! are impressed across the resistor 413 and consequently by wayof the coupling capacitors 4H and 412 across the bus 626 and theconductor 431. More particularly, the conductor 431 is connected by wayof the blade 416 and the associated break contact of the talk-listen key403, occupying its normal listen position, and the coupling capacitor4'62 to the control grid 4! I of the tetrode amplifier, whereby theelectron stream between the anode 489 and the cathode 4|2 of the tetrodeamplifier is modulated in accordance with the received undulatingcurrents. The output of the tetrode amplifier is applied from the anode609 of the tetrode amplifier through the coupling capacitor 564 and thebreak contact of the blade M1 of the talk-listen key 403 to one terminalof the transducer 453 l now functioning as a loud-speaking receiver, theother terminal of the transducer dfil being connected to the bus 426.Thus, the transducer Ail] functions as a loud-speaking receiver toreproduce at the station #2 the signals received over the associatedhome line 45i from the station #1. Also, at this time at the station #2the master switch 404 occupies its closed position, thereby renderingthe rectifier operative in a manner identical to that previouslyexplained in conjunction with the rectifier disposed at the station #1.Thus at this time the transducer 40! functions as a loud-speakingreceiver, and the tetrode amplifier functions as a beam power amplifier.

Of course it will be understood that in the intercommunicating system,after the person at the station #1 is through talking he may restore theassociated talk-listen key 303 to its normal listen position, wherebythe station #1 is converted to a receiving station; and at this time theperson at the station #2 may operate the associated talk-listen key 403to its talk position, whereby the station #2 is converted to atransmitting station. Further it will be understood that when any one ofthe stations is operating as a transmitting station, the associatedtransducer functions as a transmitting microphone and the associatedtetrode amplifier functions as a voltage amplifier; whereas when ony oneof the stations is operating as a receiving station, the associatedtransducer functions as a loud-speaking receiver and the associatedtetrode amplifier functions as a beam power amplifier.

Finally, it will be appreciated that only one station at any one timemay operate as a transmitting station, whereas normally only one stationoperates therewith as a receiving station. However, a conferenceconnection can be set up between the transmitting station and any numberof receiving stations. This is accomplished by the person at thetransmitting station making successive calls to the persons at thereceiving stations and causing the persons at the different receivingstations to operate their associated station selector switches in orderto select the home line extending to the transmitting station. When aconference connection is thus set up, the station selector switch at thetransmitting station may be released in view of the fact that the homeline extending to the transmitting station has been selected by thestation selector switches at the connected receiving stations.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood that manymodifications may be made therein and it is intended to cover in theappended claims all such modifications as fall within the true spiritand scope of the invention.

What is claimed is:

1. An intercommunicating system comprising first and second stations anda signalling channel extending therebetween; each of said stationsincluding a transducer operative selectively as a transmittingmicrophone and as a loud-speaking receiver, an amplifier provided withinput and output terminals, a switching device selectively operativebetween talking and listening ositions, and a circuit network soconnected and arranged that said first station switching device isoperative when in its talking position to connect said first stationtransducer to the input terminals of said first station amplifier and toconnect the output terminals of said first station amplifier to saidsignalling channel, and that said second station switching device isoperative when in its listening position to connect said second stationtransducer to the output terminals of said second station amplifier andto connect the input terminals of said second station amplifier to saidsignalling channel at the same time as said first station amplifier isconnected to said signalling channel.

2. An intercommunicating system comprising first and second stations anda signalling channel extending therebetween; each of said stationsincluding a transducer operative selectively as a transmittingmicrophone and as a loud-speaking receiver, a vacuum tube provided withinput and output terminals, a switching device selectively operativebetween talking and listening positions, and a circuit network soconnected and arranged that said first station switching device isoperative when in its talking position to connect said first stationtransducer to the input terminals of said first station vacuum tube andto connect the output terminals of said first station vacuum tube tosaid signalling channel, and that said second station switching deviceis operative when in its listening position to connect said secondstation transducer to the output terminals of said second station vacuumtube and to connect the input terminals of said second station vacuumtube to said signalling channel at the same time as said first stationvacuum tube is connected to said si nalling channel.

3. An intercommunicating system comprising first and second stations anda signalling hannel extending therebetween; each of said stationsincluding a transducer operative selectively as a transmittingmicrophone and as a loud-speaking receiver, a vacuum tube of thebeam-power type provided with input and output terminals, a switchingdevice selectively operative between talking and listening positions,and a circuit network so connected and arranged that said first stationswitching device is operative when in its talking position to connectsaid first station transducer to the input terminals of said firststation Vacuum tube and to connect the output terminals of said firststation vacuum tube to said signalling channel, and that said secondstation switching device is operative when in its listening position toconnect said second station transducer to the output terminals of saidsecond station vacuum tube and to connect the input terminals of saidsecond station vacuum tube to said signalling channel at the same timeas said first station vacuum tube is connected to said signallingchannel.

4. An intercommunicating system comprising first and second stations anda signalling channel extending therebetween; each of said stationsincludin a transducer operative selectively as a transmitting microphoneand as a loud-speaking receiver, an amplifier of the pentode type, arectifier connected to rectif alternating current for supplying saidpentode with direct current, a high capacity filter circuit forfiltering the rectified output of said rectifier; a separate alternatingcurrent supply connected to each of said rectifiers; an impedancenetwork for said first station for matching the input and output of saidfirst station amplifier to said first station transducer and signallingchannel, respectively; an impedance network for said second station formatching the input and output of said second station amplifier to saidsignalling channel and to said second station transducer, respectively;and means for simultaneously connecting said amplifiers to saidsignalling channel.

5. An intercommunicating system comprising first and second stations anda signalling channel extending therebetween; each of said stationsincluding a transducer operative selectively as a transmittingmicrophone and as a loud-speaking receiver, an amplifier of the pentodetype, a rectifier connected to rectify alternating current for supplyingsaid pentode with direct current, a high capacity filter circuit forfiltering the rectified output of said rectifier, a talk-listen switchfor connecting said transducer either to the input or the output of saidpentode amplifier; a separate alternating current supply connected toeach of said rectifiers; an impedance network for said first station formatchin the input and output of said first station amplifier to saidfirst station transducer and signalling channel, respectively; and animpedance network for said second station for matching the input andoutput of said second station amplifier to said signalling channel andto said second station transducer, respectively; said amplifiers beingconnected simultaneously to said signalling channel.

6. An intercommunicating system comprising a plurality of stations and asignalling channel extending therebetween; each of said stationsincluding a transducer operative selectively as a transmittingmicrophone and as a loud-speaking receiver, and an amplifier having atleast a pair of stages; one of said stages being at each of saidplurality of stations, each of said stages being adapted to functioneither as an input stage or as an output stage; impedance networksconnected to each end of said signalling channel, said impedancenetworks and said signalling channel being connected between andsimultaneously to the stages of said amplifier.

7. An intercommunicating system comprising a plurality of stations and asignalling channel extending therebetween; each of said stationsincluding a transducer operative selectively as a transmittingmicrophone and as a loud-speaking receiver, and an amplifier having atleast a pair of stages; one of said stages being at each of saidplurality of stations, each of said stages being adapted to functioneither as an input stage or as an output stage; impedance networksconnected to each end of said signalling channel, said impedancenetworks and said signalling channel being connected between the stagesof said amplifier; and means at each station for connecting the stage atthat station to function either as an input stage or as an output stage.

8. An intercommunicating system comprising a plurality of stations and asignalling channel extending therebetween; each of said stationsincluding a transducer operative selectively as a transmittingmicrophone and as a loud-speaking receiver, and an amplifier having atleast a pair of stages; one of said stages being at each of saidplurality of stations, each of said stages being adapted to functioneither as an input stage or as an output stage; impedance networksconnected to each end of said signalling channel, said impedancenetworks and said signalling channel being connected between the stagesof said amplifier; and means including a ta1klisten switch at eachstation for connecting the stage at that station to function either asan input stage or as an output stage.

9. An intercommunicating system comprising first and second stations anda signalling channel extending therebetween; said first stationincluding a microphone, a first-stage amplifier provided with input andoutput terminals, and means including a circuit network for couplingsaid microphone to the input terminals of said first-stage amplifier andfor coupling the output terminals of said first-stage amplifier to saidsignalling channel; said second station including a, loud-speakingreceiver, a second-stage amplifier provided with input and outputterminals, and means including a circuit network for coupling saidloud-speaking receiver to the output terminals of said second-stageamplifier and for coupling the input terminals of said second-stageamplifier to said signalling channel.

10. An intercommunicating system comprising first and second stationsand a signalling channel extending therebetween; said first stationincluding a microphone, a voltage amplifier provided with input andoutput terminals, and means including a circuit network for couplingsaid microphone to the input terminals of said voltage amplifier and forcoupling the output terminals of said voltage amplifier to saidsignalling channel; said second station including a loud-speakingreceiver, a power amplifier provided with input and output terminals,and means including a circuit network for coupling said loud-speakingreceiver to the output terminals of said power amplifier and forcoupling the input terminals of said power amplifier to said signallingchannel.

11. An intercommunicating system comprising a first station including amicrophone, a second station including a loud-speaking receiver, and alink interconnecting said microphone and said loud-speaking receiver;said link including a first-stage amplifier provided with inputterminals coupled to said microphone and output terminals, asecond-stage amplifier provided with output terminals coupled to saidloud-speaking receiver and input terminals, and a signal line extendingbetween said first and second stations, one end of said signal linebeing coupled to the output terminals of said first-stage amplifier andthe other end of said signal line being coupled simultaneously to theinput terminals of said second-stage amplifier.

12. An intercommunicating system comprising a plurality of stations anda plurality of signalling lines respectively extending to said stations;each of said stations including a transducer operative selectively as atransmitting microphone and as a loud-speaking receiver, an amplifierprovided with input and output terminals, a selector switch havingaccess to the signal lines extending to each of the other of saidstations, a switching device selectively operative between talking andlistening positions, and a circuit network so connected and arrangedthat said switching device is operative to its talking position tocouple said transducer to the input terminals of said amplifier and tocouple the output terminals of said amplifier to said selector switchand to the associated signalling line and that said switching device isoperative to its listening position to couple said transducer to theoutput terminals of said amplifier and to couple the input terminals ofsaid amplifier to said selector switch and to the associated signallingline.

WILLARD FELDSCHER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,954,195 Black Apr. 10, 19341,982,068 Parker Nov. 27, 1934 2,087,027 Glaser July 13, 1937 2,146,362Thomson Feb. 7, 1939 2,269,144 Campbell Jan. 6, 1942

